1. Field of the Invention
The present invention relates to an optical communications system based on a WDM (wavelength division multiplexing) technology.
2. Description of the Related Art
The transmission capacity of an optical fiber system has been increased by raising the bit rate of an optical signal from a transmission unit. Recently, there is a tendency to transmit a large volume of information using a plurality of wavelengths through the WDM technology. Therefore, it is necessary to design an upgradable communications system based on the increase in number of wavelengths to be multiplexed according to a request to increase the transmission capacity for the future.
In designing an optical wavelength multiplexing system with the upgrade taken into account, the maximum number of multiplexed wavelengths is computed. With a thus designed system, the output level of an optical signal for the entire fiber becomes high to obtain a larger interval between repeaters and a higher margin of the S/N ratio. Furthermore, with such a system having a high output level, it is necessary to set the entire output of the optical signal to an output value estimated when the system is designed to keep constant operation characteristics of the system even before the upgrade and even with a small number of multiplexed wavelengths. For example, if the system is designed to transmit eight wavelengths and it actually transmits four wavelengths before the upgrade, then it is necessary to set the entire output of the optical signals to be transmitted as the output for eight wavelengths even if four wavelengths are currently transmitted. Therefore, when four wavelengths are transmitted, the output per wave should be set larger. However, it is also necessary to reduce the output of an optical signal per wave to prevent a nonlinear effect. As a result, in addition to a normal optical signal, a compensation light is transmitted to keep constant output and reduce the output per wave.
FIG. 1 shows the undersea optical cable communications system for multiplexing the wavelengths of four waves in consideration of the upgrade for eight waves for the future.
An undersea branching unit BU 73 between stations A and B can branch a specific wavelength (xcex4), and wavelength-multiplex the branched wavelength with a plurality of wavelengths to send the branched wavelength to a branch station C using a band-pass filter. With the configuration shown in FIG. 1, repeaters 70, 71, and 72 are respectively provided in a transmission line provided between the station A and the BU 73, a transmission line provided between the BU 73 and the station B, and a transmission line provided between the BU 73 and the station C. In FIG. 1, each of the repeaters 70, 71, and 72 is provided as a single unit in each transmission line. Actually, a plurality of repeaters can be provided in each transmission line.
With the configuration shown in FIG. 1, the entire output of an optical signal can be constant and the optical output per wave can be reduced by adding a 4-wave compensation light (indicated by dotted lines) to the 4-wave optical signal (having the wavelengths xcex1 through xcex4). Each of an optical signal and a compensation light requires one OS (optical sender) per wave. In the system designed for an 8-wave transmission line, four OSs are required for compensation lights in addition to OSs for 4-wave optical signals containing information.
The system shown in FIG. 1 is designed to branch 2-wavelength optical signals and transmit them to the station C. Before the grade-up of the system, only an optical signal having the wavelength xcex4 is transmitted to the station C as a signal containing information. However, since the system is designed to transmit 2-wavelength optical signals to the station C in the system design, the output of an optical signal in the transmission line between the BU 73 and the station C cannot be set to the value defined when the system is designed if only optical signals having the wavelength xcex4 are transmitted. Therefore, one of the compensation lights transmitted from the station A is designed to be branched by the BU 73 and transmitted to the station C. As a result, the entire output level of the optical signal transmitted between the BU 73 and the station C can be set to the value defined when the system is designed, thereby securing stable system operations and reliability.
When an optical signal from the station C is wavelength-multiplexed by the BU 73 with an optical signal directly received from the station A, the system performance is deteriorated if there is a difference in power level between the signals. Consequently, it is necessary to keep an equal power level between the optical signals from the station A and the optical signals from the station C. A method of keeping an equal power level can be a method using a dummy light as disclosed by Tokuganhei 8-282822, Tokuganhei 9-208899, etc. This method is based on the phenomenon that the power level of an optical signal to be transmitted can be changed depending on the power level of a dummy light when the optical signal to be transmitted and a power-level-variable dummy light are transmitted after being multiplexed in wavelength, and then pass through the repeater 72. Thus, the power level of the optical signal from the station C can match the power level of the optical signal from the station A by adjusting the power level of the dummy light. Accordingly, as shown in FIG. 1, a main signal having the wavelength xcex4, its compensation light, and a dummy light for adjusting the power level are transmitted from the station C. The dummy light is removed by the BU 73.
As described above, if a wavelength multiplexing system is designed with the number of multiplexed wavelengths set larger for an upgraded system in the future than the value defined when the system is initiated, then it is necessary, when the system is activated, to transmit a compensation light in addition to an optical signal containing information so that the entire output of the optical signals transmitted through the optical transmission line such as an optical fiber, etc. can be kept constant. Consequently, an optical transmission terminal station requires the OSs for the corresponding number of compensation lights in addition to the OSs of optical signals having a plurality of wavelengths. As a result, the system is not desirable in cost.
The present invention aims at providing a characteristic compensation device for an optical wavelength multiplexing system for generating compensation lights without using OSs for the corresponding number of a plurality of compensation lights.
In the wavelength multiplexing optical transmission communications system, the optical communications terminal station according to the present invention includes: an optical signal generation unit for generating a main signal containing information; a light generation unit for generating a light having a power large enough to generate a nonlinear effect of an optical transmission line; and a wavelength multiplexing unit for multiplexing the wavelengths of the main signal and the light generated by the light generation unit.
The optical communications terminal station according to another aspect of the present invention includes in the wavelength multiplexing optical transmission communications system: a broad-band light source for generating a light having a broad band wavelength; and a filter for filtering a light having a predetermined wavelength. With this configuration, the terminal station wavelength-multiplexes the light, which is output from the broad-band light source and passes through the filter, with the light generated by the optical signal generation unit for generating a main signal containing information.
The method of transmitting an optical signal according to the present invention is a method of transmitting an optical signal in the wavelength multiplexing optical transmission communications system including the steps of: (a) generating a main signal light containing information; (b) generating a light having a power large enough to generate a nonlinear effect of an optical transmission line; (c) generating a light by the effect of four wave mixing from the light obtained in steps (a) and (b); (d) transmitting the lights obtained in steps (a), (b), and (c) through the optical transmission line.
The method of transmitting an optical signal according to another aspect of the present invention is a method of transmitting an optical signal in a wavelength multiplexing optical transmission communications system and includes the steps of: (a) generating a light having a broad band of wavelength; (b) passing a light having a predetermined wavelength among the lights generated in step (a); (c) generating a main signal light containing information; and (d) transmitting the lights obtained in steps (b) and (c) through the optical transmission line.
According to the present invention, it is not necessary to provide optical transmitters for the corresponding number of compensation lights to be generated because generated are the compensation lights required when the system is operated with the number of multiplexed wavelengths smaller than the number of multiplexed wavelengths estimated when the system is designed. Compensation lights can be generated by the nonlinear effect induced in the optical transmission line by the light generated by the light generation unit, or can be generated by passing a light from a broad-band light source through a filter. Therefore, the number of optical transmitters can be decreased and the entire cost for the system can be reduced.
Furthermore, the light generated by the light generation unit can be used in broadcast, and can also be used as a level control light for a main signal. A light generated by the nonlinear effect induced by the light generated by the light generation unit is the same optical signal as the main signal. Therefore, it can be used as a spare signal for the main signal. When a compensation light is generated by passing a light from a broad-band light source through a filter, the compensation light does not contain any signal. Therefore, the generated compensation light can be modulated for broadcast. Otherwise, an optical signal, which is generated by the nonlinear effect induced by a light generated by the light generation unit and contains the same information as the main signal, can be used to detect an error in the main signal.